1. Field of the Invention
The present invention is related in general to flash tube devices and in particular to prolonging the ability of a flash tube to produce light with a preferred intensity.
2. Description of the Related Art
Flash tubes have been used in many applications, such as photography, photocopying, aircraft traffic control and stroboscopes, to provide flashes of high intensity light for film development, guidance and the like. A flash tube is generally formed by an envelope, such as a tube made of quartz or silicon dioxide, which encloses two electrodes and a small amount of noble gas such as Xenon. Typically, the electrodes are connected in parallel with a capacitor, which will be charged to a high voltage during operation. As the capacitor is charged, the voltage across the electrodes will increase. However, because the gas normally has a high resistance (typically at 10 megohm), no electric current will flow across the electrodes until a high voltage pulse is applied to a trigger electrode of the tube, at which time the gas will be ionized. When the gas is ionized, its resistance is reduced significantly (typically down to about 1 ohm) and the electric charge stored in the capacitor will discharge through the ionized gas, emitting light.
Along with the light, heat is emitted during the discharge. The heat raises the temperature of the flash tube and often creates cracks in the envelope, through which the gas may escape. The high electrical voltage applied to the electrodes may also cause the electrodes to wear, a process commonly known as "electrode pitting". Leakage of the gas through the cracks, and pitting of the electrodes, will cause degradation in the performance of the tube, with the result that the intensity of the light emitted from the flash tube will gradually decrease even though the same amount of the electrical energy is supplied to the tube. Performance of the flash tube will also degrade when the capacitor, particularly the dielectric material thereof, deteriorates over time. The deterioration is faster when the operational voltage of the flash tube is increased.
For applications where a predetermined amount of light is expected, degradation of the flash tube may produce undesirable results. For example, one such application is disclosed in U.S. Pat. No. 5,151,595, where a flash with a duration of 200 microseconds and beam intensity of 1500 BWPS (beam watt per second) is preferred for operation of an imaging device. If the operating voltage of the flash tube is set to produce 1500 BWPS exactly, degradation of the flash tube would adversely affect effectiveness of the device, as the intensity will subsequently decrease to below 1500 BWPS. To allow for such degradation, the voltage across the electrodes can be increased to set the initial light intensity at a level substantially higher than the required 1500 BWPS (i.e., to increase the margin of the intensity) so as to prolong the time in which the degradation will cause the intensity to fall below the required 1500 BWPS. However, the rate in which the flash tube degrades will increase when the voltage across the electrodes is increased, because a higher voltage will accelerate pitting of the electrodes and deterioration of the capacitor. Moreover, operating the flash tube at a higher voltage will raise the temperature of the flash tube, which in turn will accelerate cracking of the envelope.
What is needed is a technique for prolonging the effective life span of the flash tube in producing light with a preferred intensity.